<svg xmlns="http://www.w3.org/2000/svg">

  <mask id="m">
    <text>
      <tspan id="tspan" />
    </text>
  </mask>

  <rect width="600" height="400" mask="url(#m)"/>

  <script>

    window.addEventListener("load", function() {
      document.getElementById("tspan").style.dominantBaseline = "alphabetic";
    }, false);

  </script>

</svg>
